A defibrillator can be used to restore a normal heart rhythm by delivering an electrical shock to the heart when the heartbeat is dangerously fast due to ventricular tachycardia or ventricular fibrillation. Either of these conditions can reach a life-threatening point at which a person suddenly loses consciousness because the heart can no longer pump enough blood to meet the body's demand. For patients suffering from chronic arrhythmias involving ventricular tachycardia or ventricular fibrillation, a defibrillator can be surgically implanted in the patient's chest. The implanted defibrillator can be implanted into the chest of the patient during a minor surgical procedure.
An implantable cardioverter defibrillator (ICD) is a device that can be implanted in a patient's chest to monitor for and, if necessary, correct episodes of rapid heartbeat. If the heartbeat gets too fast (ventricular tachycardia), the ICD can stimulate the heart to restore a normal rhythm. In cases where the heartbeat is so rapid that the heart cannot effectively pump any blood (ventricular fibrillation), the ICD can provide an electric shock to “reset” the heartbeat.
The ICD gets its name from the two functions that it performs. First, the ICD sends small electrical charges to the heart to “reset” it during ventricular tachycardia. This process of converting one rhythm or electrical pattern to another is called cardioversion. Second, the ICD will send stronger charges to “reset” the heart if it begins ventricular fibrillation instead of beating. The act of stopping this potentially fatal quivering of the heart is called defibrillation. Although the main functions of the ICD are cardioversion and defibrillation, it can also be programmed to do anti-tachycardia and bradycardia pacing.
In anti-tachycardia pacing, when an ICD senses a fast but rhythmic heartbeat (tachycardia), it can release a series of low-intensity electrical pulses that gently interrupt the heart and allow it to return to a slower pace. In bradycardia pacing, when the ICD senses an abnormally slow heartbeat, it can send small electrical signals to pace the heart until it recovers and maintains a normal heart rate. These therapies are contrasted with both cardioversion and defibrillation, which involve high voltage shocks, which is the focus of the present invention.
In all of the ICD systems available today, a truncated capacitive-discharge shock is delivered by the ICD to electrodes that are positioned in, on, or near the heart. To generate the shock, existing ICD systems use an internal high current electrical battery cell connected to a step-up transformer and power conversion circuitry to charge one or more relatively small, but powerful, high voltage capacitors to provide a relatively high discharge voltage. When an electrical stimulation pulse is to be applied to the heart, the appropriate output switch is closed to connect the output capacitor to the cardiac tissue through the electrodes, thereby effectively “dumping” the charge stored in the output capacitor into the cardiac tissue. After the output decays to a predetermined output voltage, or after a predetermined shock duration has elapsed, the shock is truncated and the remaining energy in the output capacitor system is dissipated within the ICD system never being utilized or recovered.
The primary function of an ICD is to sense the occurrence of an arrhythmia, and to automatically apply an appropriate shock therapy to the heart aimed at terminating the arrhythmia. For example, if the ICD senses that the patient's heart is fibrillating then the ICD automatically delivers a high current shock to the patient's heart to defibrillate the organ. ICDs typically operate by first detecting the arrhythmia, then rapidly charging one or more storage capacitors contained within the device, and then quickly discharging the capacitor(s) to deliver the life saving shock therapy. However, a problem associated with rapidly charging a capacitor is that it creates a severe load on the battery. Thus reducing the battery's life.
An additional problem associated with the high voltage capacitors of an ICD is the amount of time it takes to charge the capacitors, typically about 5 to 20 seconds. Many studies have proposed that defibrillation and cardioversion shocks are most effective when delivered as quickly as possible following detection of arrhythmia. The chance of terminating an arrhythmia in a patient decreases as the length of time it takes for therapy to be delivered to the patient increases. Therefore, the shorter the charge time for the capacitors the more effective the defibrillation therapy. Typically, ICD battery sizes are proportional to the charging time. Therefore, the quicker the desired charging time, the larger the battery. In spite of this, it is desirable to make the ICD as small as possible and therefore large batteries are not desired and thus a balance must be struck between having a fast charging time and the size of the ICD.
Another problem involves providing a capacitor that maintains a high capacitance while at the same time has a reduced leakage current. The term “leakage current” refers to the measure of stray direct current flowing through a capacitor after DC voltage is impressed on it and is expressed in milliamps.
The dielectric of a capacitor has a very high resistance, which prevents the flow of DC current. However there are some areas in the dielectric, which allow a small amount of current to pass. The value of leakage current will continue to decrease while voltage is applied to the capacitor, until a very low steady state leakage current value is reached. However, as stated above, the present ICDs allow the remaining capacitor charge to dissipate after the arrhythmia has been treated. The longer capacitors are stored with no applied voltage, the higher the initial leakage current. Therefore, the constant recharging and the length between the recharging of the capacitors actually increases the amount of leakage current. A high leakage current can result in the poor performance and reliability of a capacitor. In particular, high leakage current results in a greater amount of charge leaking out of the capacitor once it has been charged. This is undesirable.
Another problem associated with the present ICDs, is that the remaining charge after the arrhythmia is treated is just dissipated within the ICD. While the charge dissipated is relatively minimal when compared to the shock charge, after hundreds of shocks the remaining charges can add up to a substantial shock. Typically, 16 remaining charges can add up to provide a defibrillation shock. Further, the dissipated remaining charges equate to energy taken from the battery and never put to use. Therefore, it would be desirable to capture these remaining charges and thus extend the life of the battery.
The discussion now turns to an ICD therapy, referred to as a high-power therapy, that delivers energy to a patient in approximately 10 milliseconds (ms). High-power therapy uses defibrillation capacitors at high energy defibrillation pulses (e.g. 0.1-35 joules (J)). The battery that powers the ICD does not directly provide energy to the patient's tissue. Instead, the ICD battery charges a high-energy, high-power capacitor system. High-energy, high-power capacitor system are also referred to as high-voltage therapy capacitor(s), main energy delivery capacitor(s), high-power capacitor(s), or other similar names. To date, all marketed ICDs use either aluminum or tantalum electrolytic capacitors for high-power therapy.
The amount of energy delivered by the capacitors is controlled by the voltage to which the capacitors are charged. The highest voltage to which the capacitors can be charged corresponds to the maximum energy therapy. The highest voltage typically relates to a few volts below the maximum rated voltage of the capacitors.
Electrolytic capacitors exhibit high leakage currents when operated near their maximum rated voltage. To minimize excessive power consumption, the high-voltage therapy capacitors are not maintained in a continuously charged state, but rather are charged only when an episode occurs. An episode is defined as the time period in which the ICD determines that a high-voltage therapy is required. Between episodes, the capacitors are allowed to rest uncharged. In the uncharged state, the charging efficiency of electrolytic capacitors degrades. Consequently, when the capacitors need to be charged at a later time, more energy and longer charge time is required. Therefore, ICDs are typically programmed to periodically charge the high-voltage capacitors in order to achieve charging efficiency. This process is referred to as reformation, as it is thought to “reform” the anodic oxide. Skilled artisans generally consider reformation as requiring that the capacitors be charged to their maximum rated voltage or the maximum energy voltage of the ICD. Charging the capacitors to their maximum rated voltage is referred to as the nominal reformation voltage of the device.
U.S. Patent No. 5,620,464 issued to Kroll et al. exemplifies a conventional process of periodically charging the capacitor without a continuous charge being applied to the high-voltage therapy delivery capacitor. In Kroll, the main energy delivery electrical circuit depicted in FIG. 6 for use in an ICD comprises a low power output primary defibrillator battery, a high power output intermediate power intensifying capacitor system, a switch for permitting the intermediate power intensifying capacitor system to rapidly charge a main energy delivery capacitor, and a main energy delivery capacitor. The main energy delivery capacitor is configured for discharging, in a first pulse, an electrical charge derived from the primary battery, and for discharging certain subsequent pulses of electrical charge derived from the intermediate power intensifying capacitor system. The circuit permits the ICD to deliver multiple closely spaced defibrillation pulses to a heart. The power intensifying system is periodically recharged from the primary power source. Kroll defines the power intensifying capacitor system as being separate and distinctly different in function from the main energy delivery capacitor. Kroll also specifies the types of energy storage devices which are suitable for the power intensifying system. Kroll is not suitable for use as a main energy delivery capacitor (i.e. therapy delivery capacitor).
For the foregoing reasons, there is a need for an ICD, which allows for a relatively long charging time and yet retains clinical efficacy to prolong battery life and provide for a smaller battery. There is also a need for an ICD providing a high voltage capacitor with very low leakage current so that the capacitor could be held at full charge thus reducing the adverse effects of rapid charging. There is also a need for an ICD that when an arrhythmia is detected the ICD can deliver therapy at the quickest possible moment without having to wait for a capacitor to charge thus increasing the efficacy of the delivered therapy.